The invention relates to diffusion pumps for producing high vacuum and more particularly to an improved diffusion pump by which the pump fluid may be separated from its impurities during operation.
Diffusion type pumps for producing high vacuum are well known. The original effort of Gaede around 1914 was supplemented by Langmuir in the development of the vertical jet diffusion pump disclosed in U.S. Pat. No. 1,393,350, and the single inverted jet or mushroom pump covered by U.S. Pat. No. 1,320,874. A multiple jet modification of the mushroom pump is described in U.S. Pat. No. 1,367,865. Such pumps operate on the principle that a liquid having relatively heavy molecules is vaporized in the pump by raising its temperature. The vapor comprising heavy molecules is directed by suitable nozzles in a direction away from the region to be evacuated, towards a mechanical forepump. The accelerated molecules of vapor compress against molecules ahead of the nozzle, forcing them toward the mechanical forepump and thereby reducing the pressure within the evacuated region. The vapors are recondensed on a cool wall of the pump where the liquid is permitted to return to the bottom of the pump to be reheated and vaporized.
Originally mercury was employed as the working fluid in these diffusion pumps, but later various organic oils and silicone fluids were developed as pump fluid, and today these fluids have almost completely replaced mercury in diffusion pumps. In particular, the silicone oils, of which DC-705 (pentaphenyl trimethyl trisiloxane) manufactured by Dow Corning Corp., is an example, are in wide use today.
Performance of diffusion pumps has been erratic and subject to certain limitations. It was long ago observed that the evaporative surface of a studied pump fluid in a pot still, when rapidly evaporating, seems to separate into two different areas of turbulence, resulting in a "schizoid" evaporative surface. In one area of the surface, termed the "working" area, very rapid evaporation of the fluid takes place, while in the other area, known as the "torpid" area, very little evaporation takes place. A discussion of this phenomena is found in the article "Torpid Phenomena and Pump Oils", K. C. D. Hickman, The Journal of Vacuum Science and Technology, Volume 9, No. 2, and "Surface Behavior in the Pot Still", K. C. D. Hickman, Industrial and Engineering Chemistry, Volume 44, No. 8. Since the torpid areas of the evaporative surface within the diffusion pump boiler release vapors at a very low rate, the diffusion pump speed, throughput and ultimate vacuum attainable are limited to the extent that the evaporative surface is affected by torpidity.
Various remedies have been suggested to alleviate or overcome the problem of torpidity in diffusion pump boilers. See, for example, the above article by Hickman, Hickman U.S. Pat. No. 2,080,421, and "A New Type of Diffusion Pump Boiler for Ultrahigh Vacuum Use", H. Okamoto and Y. Murakami, Vacuum, Volume 17, No. 2. The suggested solutions include the use of a central purge sump within the boiler for segregating certain impurities which overflow therein during boiling; the use of a boiler heater designed to induce tremendous turbulence in the pump fluid (Stevenson Flash Boiler); and various means for circulating the pump fluid in the boiler. The latter means include stirring or otherwise rotating the fluid mechanically, and positioning the applied heat so as to induce circulation (N-boiler of Murakami). Numerous diffusion pump boiler modifications are shown and described at pages 974-976 of the first above-referenced Hickman article.
While then suggested solutions have reportedly increased molecular throughput somewhat, they do not have the capability to purify the pump fluid within the boiler thereby eliminating the causes of torpidity, as discussed below. The exception is the purge sump, which does remove a limited quantity of impurities from the surface of the evaporating liquid. As long as torpid areas of the evaporative surface prevail, molecular throughput and attainable vacuum remain drastically limited.
Another limitation in diffusion pumps on ultimate attainable vacuum is imposed by a phenomenon known as "backstreaming". Backstreaming, also known as "reverse fractionation", constitutes a back migration of some molecules from the jets back into the vacuum chamber and is inherent in a diffusion pumping process. As pressure in the chamber being evacuated decreases, the rate of backstreaming increases, and when it equals the throughput of gas, no further decrease in chamber pressure occurs. The phenomenon of backstreaming, and various suggested remedies therefor, are discussed in Hickman U.S. Pat. Nos. 3,034,700 and 2,080,421, Scatchard U.S. Pat. No. 2,905,374, Nelson U.S. Pat. No. 2,291,054, Bachler U.S. Pat. No. 3,317,122 and Hayashi U.S. Pat. No. 3,171,584. For example, in Hickman U.S. Pat. No. 3,034,700 and in Bachler U.S. Pat. No. 3,317,122 it is suggested that backstreaming can be reduced by cooling the diffusion pump barrel only behind the jet or adjacent the upper stages of a multi-stage jet assembly, with the lower portions of the diffusion pump barrel being maintained warm. This reportedly maintains a long column of forwardly moving pump fluid vapor, giving the molecules less chance to diffuse backstream. Another often employed way of reducing backstreaming is the use of one or more cryogenically cooled baffles between the vacuum chamber and the pump. The baffle primarily attempts to condense and trap contaminant gases from the chamber and to prevent diffusion pump vapors from backstreaming into the chamber. Many of the chamber gases originate from materials in the chamber which have "outgassed" under the influence of high vacuum. While cooled baffles have been helpful in trapping chamber gases and reducing backstreaming, they have not been able to trap all passing gases, and once they are filled with condensate, they lose their effectiveness. If the baffle is warmed, the condensables drip into the boiler and cannot be removed.
Although molecules of the pump fluid itself exiting the diffusion pump jets have a tendency to backstream to a slight degree, it is primarily molecules of "light ends" which backstream through the diffusion pump barrel toward the lower pressure vacuum chamber, thus severely limiting the degree of vacuum attainable. "Light ends" are those contaminants present with the pump fluid which are of lower molecular weight than the pump fluid itself, and may include broken away fractions of molecules of the pump fluid itself, which is, in the case of the pump fluid DC-705, a pentamer. One system for partially removing light end contaminants from the pump fluid, thereby reducing backstreaming, is shown in Hickman U.S. Pat. No. 3,034,700, FIG. 1, and in the first above-referenced Hickman article, FIG. 25 and page 976. This system consists of collecting condensed distillate in annular alembics defined in the foreline of the diffusion pump. The distillates comprise light end substances which have escaped the diffusion pump barrel and passed along with vacuum chamber gases into the foreline.
In Hickman U.S. Pat. No. 2,080,421, wherein total pump fluid constituents, including impurities, are identified by letters A through Z from the lightest light ends through the heaviest ends, diffusion pump apparatus is disclosed wherein certain impurities were isolated within internal compartments. However, only contaminating components A, B and Z are disclosed as having been successfully isolated.
While the diffusion pump structures discussed and referenced above aid in the reduction of torpidity on the fluid's evaporative surface within the boiler and in the reduction of backstreaming by light end substances into the vacuum chamber, thereby increasing ultimate attainable vacuum, the suggested structures cannot produce a 100% continuously working evaporative surface, nor reduce backstreaming and achieve high fluid separation to the extent of the present invention described below.